Count rate performance and deadtime analysis of the new 3D PETRRA PET Camera K.Wells *a, C. Kakanab, R.J. Ott **c, M. A. Flowerc, A Divolic, S Meriauxc, J.E.Batemand, R Stephensond, D Duxburyd , E Spilld a School of Electronics, Computing & Mathematics, University of Surrey b School of Physical Sciences, University of Surrey c Joint Dept of Physics, Royal Marsden NHS Trust Hospital/Institute of Cancer Research d Rutherford Appleton Laboratories
ABSTRACT We report on the count-rate performance of the unique PETRRA positron camera at activities up to 60MBq. The camera consists of two large area detectors, each comprising a tiled array of 10mm thick BaF2 scintillation crystals interfaced to a multi-step avalanche chamber filled with 4.2mbar of pure TMAE vapour. Preliminary results demonstrate coincident count rates of over 80kcps for a cylindrical (20x20cm3) phantom with 50MBq of F-18 in the field-of-view using a 20ns coincidence time window. Each component of the readout cycle has been characterised in terms of dead-time loss. The camera’s dead-time related count loss is well-described by a paralysable model with a dead-time of ~500ns. Other sources of count rate loss are also discussed. Keywords:
PET, PETRRA, count rate, dead-time, BaF2, TMAE
1. INTRODUCTION Over the last two decades there have been a number of attempts to harness multi-wire proportional chamber (MWPC) technology for use medical imaging, in particular, Positron Emission Tomography (PET). Among these, the HIDAC system[1] and the MUP-PET system[2] have been the most well-known. Within the Institute of Cancer Research/ Royal Marsden NHS Trust Hospital, the MUP-PET system has shown that clinically useful PET imaging can be achieved using this technology[3]. This system has also shown the benefits of using true 3D PET reconstruction and imaging. However, the very modest sensitivity of the system has proved to have severe consequences for imaging. We have therefore been developing a new MWPC PET system, based on BaF2-TMAE (Tetrakis (dimethyl-amino)ethylene) technology originally developed for use in high energy physics calorimetry[4] . Detection of annihilation radiation in BaF2-TMAE detectors is a two stage process. Energy deposition by ionising radiation, generates UV scintillation light in the BaF2, which when emitted, is then absorbed in the sensitive TMAE vapour. Successful detection of events relies on the overlap of the BaF2 ultraviolet scintillation spectra with the TMAE absorption spectra[4]. TMAE also has a low ionisation threshold (7.5 eV) which means that the absorbed energy of the scintillation light can be efficiently converted into energetic free photoelectrons. These primary conversion electrons are then gas multiplied in a multi-step gas avalanche chamber (MSA) and position determined using conventional MWPC readout techniques. The PETRRA BaF2-TMAE system has been in development for a number of years, progressing from bench top hybrid through to full-size dual head camera system[5,6]. The final system consists of two gantry mounted, large area (60x40cm2) detectors that rotate about 1800 . Clinically, the main advantage of this technology is the 40cm field-of *
[email protected]; phone 44-1483 686036; fax 44 1483 686031; http://www.ee.surrey.ac.uk/CVSSP; School of Electronics, Computing & Math, University of Surrey, Guildford Surrey, GU2 7XH, UK **
[email protected]; phone 44-208-642 6011; fax 44 208 643 3812; http://www.icr.ac.uk; Royal Marsden Hospital /InstituteofCancerResearch, Sutton, Surrey SM2 5PT UK
Penetrating Radiation Systems and Applications III, H. Bradford Barber, Hans Roehrig, F. Patrick Doty, Richard C. Schirato, Edward J. Morton, Editors, Proceedings of SPIE Vol. 4508 (2001) © 2001 SPIE · 0277-786X/01/$15.00
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view, allowing large organs such as lung or liver to be imaged without patient movement. We now report on further detailed tests of count-rate performance and the various losses in the electronic read-out system.
2. DETECTOR STRUCTURE Fig. 1 shows a cross sectional schematic diagram of the main elements of the two large area detectors. Visvikis[7] discusses the construction of the first large area detector, whilst Duxbury [8] discusses some of the further engineering features of the technology. Below the tiled array of BaF2 scintillation crystals is a MSA structure divided into 3 main regions: the primary detection region, the gated transfer region and the coincident readout section. The primary detection region is divided into six parallel sectors, each with its own preamplifier-discriminator unit. The output from each of these units is used to control the opening of a corresponding section of the gate structure within the transfer region. This minimises deadtime losses at the front of the detector, and also minimises corruption in the readout cycle, which is discussed in more detail in section 3. Within the absorption region approximately 1-2 photoelectrons are detected per gamma photon interaction. Although there are intrinsically around 5-6 photoelectrons produced per 511keV deposited, the structure of the chamber is such that only the first one or two photoelectrons created in the absorption region are likely to undergo sufficient gas multiplication to ensure successful completion of the readout cycle. These initial photoelectrons are drifted across a high electric field initiating a small amount of gas multiplication. Further, more substantial, gas multiplication occurs in the preamplification region. As the resulting electron shower passes out of the primary conversion region, their passage is sensed by one of the six preamplifier/discriminator modules coupled to the final wire plane in the primary detection region. This is used to arm one side of a coincidence unit. If a corresponding event is found in temporal coincidence in the second detector, then an enable signal is produced to initiate the readout cycle. A screened gate structure is positioned in the middle of the transfer region that efficiently blocks unwanted singles events from passing into the readout cycle. The gate is also segmented into six parallel sections corresponding to those
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in the primary detection region. Assuming a coincidence event has been detected in both detectors, then the electric fields within one of the corresponding gate regions is pulsed for 100ns to allow free passage of the electron cloud through to the MWPC readout section. Position readout is accomplished using orthogonal delay lines coupled to the x and y MWPC cathodes. Constant fraction discriminators (CFDs) connected to Time to Analogue Converters (TACS) determine the difference in arrival times of the pulses at the end of each delay line. The resulting data is digitised and used to compute the x,y position of the coincidence event in each detector. Before passing the coordinate data to the computer system several checks are made to ensure that the data has not been corrupted. Assuming the data is valid, then a flag is set initiating data transfer to the computer system.
3.
COINCIDENCE COUNTING
Counting events in time coincidence has a number of implications for the way in which pulse information is handled. We now describe some of the main issues for coincidence counting. For further details, refer to Bateman [8] and Visvikis[9] . The finite time, τ, taken to process an analogue pulse in any practical counting system means that there are inevitable losses of the true count rate, R, due to the insensitivity of counting electronics whilst this processing takes place. This fractional loss of events, F, becomes particularly serious at high data rates and can usually be characterised by one of two models[9]: either the paralysable (sometimes referred to as extendable) model, described by [1] below, or the nonparalysable (or non-extendable) model described by [2], Fp = e - R
τ
Fn = 1 / (1 + Rτ)
[1] [2]
Although exhibiting similar behaviour at low rates, where counting losses are ≤10% level, at higher rates the paralysable model will describe an ever decreasing event rate tending to zero, following a maximum at 1/τ, whilst the non-paralysable model will tend to approach a value of Rτ at high count rates. Further details may be found in [9] . The observed singles count rate of each sector, No, in the primary detection region can be related to the actual singles rate, Ns, as No = Ns F1
[3]
where F1 is the count rate dependent deadtime factor for the CFDs, described with reference to one of the two models in [1] or [2] above. Assuming that each sector sees approximately the same solid angle flux of photons, then the overall effective deadtime , τ, is effectively reduced by ~factor 6 due to the segmentation of the primary detection region. The total observed coincidence counting rate registered in the primary detection region is given by the sum of the fast (trues + scatter) coincidences Nf, and the random or accidental coincidence rates Nr: Ntot = Nf + Nr .
[4]
Once the electron shower has been recognised as coincidence event, and so passes from the primary detection region to the MWPC for readout, there is a chance for some of these electrons to be lost due to, for example, slight mistiming with the opening of the gate field. This may produce a loss of events across the transfer region as the attenuated shower may fail to trigger the anode discriminator. For a given detector, this non-rate dependent loss, referred to as L1, can be
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determined from : Na = L1(Ntot + Fg) ,
[5]
where Na is the counting rate recorded at the detector’s anode plane. The extra term Fg appears because whilst the gate is open, there is also a chance for unrelated avalanches which are temporally coincident with the gate opening time to also pass through into the MWPC. In practice it can shown that for gate opening times of 100ns, and given that each front sector sees approximately 1/6th of the singles photon flux, then Fg (≈ NtotNs x 100ns / 6) [6] is expected to be of the order of a few per cent of Ntot. Successful readout of x,y positional information is subject to two further sources of loss. The first of these, L2 , refers to non-rate dependent losses from incomplete coordinate sets. This may be due to pulse attenuation in the delay lines causing a failure to trigger one of the CFDs. As with L1 , L2 is also a non-rate dependent loss. The final source of loss, F2 describes rate-dependent event corruption due to extra pulses in the delay lines (such as arising from the term Fg in [5] above). So assuming Na is similar in both detectors, the final coincidence count rate passed to the data acquisition computer, RDAQ is given by RDAQ = [L2 ] 2F2 Na
[6]
The squaring arises because RDAQ is the result of using two independent detectors. Note that F2 is not squared because the pile-ups caused by deadtime are synchronised in both detectors as a result of counting coincidence events.
4. 4.1
METHODS & MATERIALS
DECAYING SOURCE METHOD
In order to determine the dead time and other losses associated with the PETRRA detector and counting electronics, we have used the classic decaying source method. A full description of this method is contained in Knoll[9] , but we summarise the main points below. Assuming that the background counting rate of a counting system is negligible, we can measure the temporal decay of a short-lived radioactive source by recording an initial count rate, Ni, and then systematically measuring the drop in count rate, N(t) at some time t later (where λ is the decay constant associated with the particular source): N(t) = Ni e -
λt
[7]
By combining this with the non-paralysable model, we obtain N(t) e
λτ
= -NiN(t) τ + No
[8]
By similar reasoning we can obtain an expression for the paralysable case:
λt + ln N(t) = -Ni τ e
−λt
+ lnNi
[9]
The dead time, τ, of the component under investigation is determined using a straight-line plot from [8] or [9], where the gradient will be equal to -Niτ. 4.2
EXPERIMENTAL TECHNIQUE
In order to study the count rate performance and deadtime characteristics of the PETRRA camera system, we uniformly
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filled a 20cm x 10cm cylindrical phantom with Fluorine-18. Before dilution, the aliquot containing the activity was placed in a calibrated well counter. The absolute activities at various points during the experiment were then determined by extrapolation from this initial reading. The phantom was placed in the centre of the field of view of the two large area detectors separated by 95cm between crystals. Over a period of hours the singles and coincidence count rates were recorded from different points in the readout system. The random coincidence component was calculated using [5], with a coincidence window of 20ns.
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5. 5.1
RESULTS & DISCUSSION
SINGLES COUNT RATES
Fig. 1 shows the mean uncorrected singles count rates for the 10cm x 20cm cylindrical phantom along with the separate count rates for the two detectors. Clearly there is some different in performance, with detector 1 decreasing in count rate slightly at activities above 35MBq. By contrast detector 2 shows a more slowly varying loss of count rate. The decaying source method was employed to investigate the dead time behaviour of both detectors, and analysed by fitting both models to the singles data. Figure 2 shows the effect of applying the non-paralysable model to the data for detector 1 using a least squares fit. This clearly shows a poor fit, with similar behaviour exhibited by detector 2. By contrast, Fig. 3 shows the result when a paralysable model is applied to the data for detector 1. This visually shows a very good fit to the data, with a corresponding r2 value of .999. This leads to an experimentally determined value for the deadtime of 500ns. When considered that this represents approximately one sixth of the total count rate, this leads to an estimate for the singles deadtime of 3 µs. This is a rather surprising result, as the pre-amplifier discriminator combinations are expected to behave an nonparalysable sources. It has been suggested that this apparent paralysable behaviour may be due to a loss of detector gain at high counting rates due to slow ion clearance at the front of the detector[10] and possibly in part due to saturation induced deadtime in the front preamplifiers[7].
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It is impossible to analytically solve [1] to correct for the singles dead time loss. Instead, we resort to using a Taylor expansion as an approximation [10] , so that from [7], ignoring higher terms, we may write: τ
R /Ro =e - R ≈ 1 + (-Rτ) +
......
[10]
Fig. 4 shows the effect of correcting the raw data from detector 1 using the above approximation. The predicted singles were estimated by extrapolation from data occurring at low count rates, where dead time was thought to be insignificant. This shows that use of the Taylor expansion provides a good correction up to approximately 28MBq in the field of view, equivalent to singles count rates up to approximately 1.2 Mcps. At higher count rates and activities, the approximation underestimates the actual singles rates as higher terms of the expansion become significant. Thus, the Taylor approximation is only suitable for correcting paralysable behaviour when Rτ90kcps for a 20 x 10cm uniformly filled head phantom. However, losses in the counting system reduce this to approximately 65kcps. This work has highlighted the efficient functioning of the MSA part of the PETRRA detectors. However, the dominant sources of event loss is associated with the primary detection region, where >50% loss in counting rate is experienced at activities >>30-35MBq. Future modifications to the design of the detectors and front-end electronics should help to reduce these losses. If such improvements can be made then we confidently expect to be able to realise recorded coincident event rates >100kcps. However, these results do show the benefits of using a segmented gated detector. The division of the primary detection region into effectively six independent counting units minimises electronic deadtime losses at the front of each large area detector. Dividing the gate into 6 sectors reduces to insignificant levels the fraction of unwanted singles showers transmitted through to the MWPC. Thus, whilst the front of the detector is counting at the full singles rate, the gate situated midway through the transfer region can block unwanted uncorrelated electron showers and only allow through the desired coincidence events, sparing the MWPC from singles saturation. This means the back of the detector, which has an intrinsically lower count rate capability due to the readout system, counts at approximately 10-1 slower compared to the primary detection region at the front. These results have then demonstrated that a large area detector can be read out efficiently, without significant corruption of singles events, which have been seen to be a significant handicap in the previous large area MUP-PET system [2,8]. It is worth noting that had the PETRRA system been developed without the segmented gate structure, then despite much improved detection efficiency (~30% compared to 7% in MUP-PET), the singles could be expected to saturate the readout electronics at coincidence rates of around 15-20kcps. This is in stark contrast to the >80 kcps coincidence rates which have been observed in the current PETTRA system, demonstrating the enhanced count rate capability of segmented gated readout at higher levels of activity. It should also be noted that the deadtime of the TAC system could be reduced substantially (to
